1. Field of the Invention
The invention relates to an improved three-order sigma-delta modulator, which implements a feedback and a feedforward configuration, so that the desired noise transfer function is carried out by adjusting the coefficients of the feedback and the feedforward.
2. Description of the Related Art
As well-known in a sigma-delta modulator, Signal to Noise Ratio (SNR) is dependent on the oversampling ratio, the sigma-delta modulator order and quantizer resolution. Theoretically, doubling the oversampling ratio should result in a 3 dB improvement in SNR, and increasing the order of the sigma-delta modulator by one should produce another 6 dB improvement in SNR within the desired bandwidth. A 1-bit (two value) increase in quantizer resolution should yield another 6 dB. Accordingly, with proper design, the sigma-delta modulator can have the advantages of high resolution, simple circuit, no additional trimming circuit and high tolerance for device replacement. Therefore, such a sigma-delta modulator is widely applied in situations requiring higher SNR, for example, by digital video disk players. As shown in FIG. 1, a typical three-order or more sigma-delta modulator feeds back the digital signal Y from an Analog to Digital (A/D) quantizer 14 to the input terminals respectively to the integrators 11, 12 and 13 through a D/A converter 15, so as to form a desired three-order or more noise transfer function. Such a structure can achieve different noise transfer functions by adjustment of the integrator's output coefficients. However, the feedback configuration in the sigma-delta modulator will cause overload and require complicated circuitry when implementing the desired noise transfer function, even though it can have higher resolution and higher SNR with higher orders.